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T4000 SONET Clock Generators (SCGs) Description


SCG Slots

The SCGs are installed into the upper rear of the chassis in the slots labeled SCG0 and SCG1.

SCG Function

SCGs provide 19.44-MHz Stratum 3 clock signal for the SONET/SDH interfaces on the router.

SCG Redundancy

One SCG is shipped as part of the standard router configuration, but up to two SCGs can be installed to provide redundancy. A nonredundant SCG is hot-pluggable. For redundant SCGs, the master SCG is hot-pluggable. If the master SCG is functioning, the backup SCG is hot-removable and hot-insertable. Removing the backup SCG does not affect the functioning of the router. Taking the master SCG offline might result in a brief loss of SONET clock lock while the backup SCG becomes the master.

Supported SCGs

The T4000 router supports the SCG in Table 1.


Redundant SCGs must be the same model number, except during upgrade.

Table 1: Supported SCG


Model Number

First Supported Junos OS Release

SCG with RJ-48 ports



SCG with DB-9 ports



SCG Components

The SCG with DB-9 ports is shown in Figure 1.

Figure 1: SCG with DB-9 Ports
SCG with DB-9 Ports

The SCG with RJ-48 ports is shown in Figure 2.

Figure 2: SCG with RJ-48 ports
SCG with RJ-48 ports

Each SCG consists of the following components:

  • 19.44-MHz Stratum 3 clock.

  • Field-programmable gate array (FPGA) that performs multiplexing of clock sources.

  • SCG online/offline button, located on the SCG faceplate.

  • Two external clock inputs.


    The external clock ports are not supported on the SCG with DB-9 ports.

    For information about configuring external clock synchronization for T Series routers, see the Junos OS Administration Library.

    For information about configuring an interface transmit clock, see the Junos OS Network Interfaces Library for Routing Devices.

  • Three LEDs—OK, FAIL, and MASTER, located on the SCG faceplate, that display the status of the SCG.